Microvascular occlusion by poorly deformable erythrocytes is believed to be the key pathophysiologic event in sickle cell disease. A quantitative understanding of the molecular and cellular processes involved in the loss of deformability of sickle erythrocytes as compared to normal erythrocytes is crucial for clarification of the pathophysiology and treatment of this disease. Polymerization of hemoglobin S (Hb S) within sickle erythrocytes is presumably one of the most important factors in the loss of deformability. The main determinant of Hb S polymer fraction in any population of sickle cells is oxygen saturation, and polymer fraction can be calculated as a function of oxygen saturation. However, the exact relationship between the deformability of intact erythrocyte and the formation of intracellular Hb S polymer is not yet well understood. On the other hand, both the abnormal membrane and the very high intracellular Hb concentrations in the populations of dense sickle cells also contribute to the rigidity of these cells. The relative contributions of these factors to cell rheology, as compared to polymerization, is also unknown. We, therefore, investigated the effects of these three factors: intracellular Hb S polymerization, the abnormal membrane, and the increased total intracellular Hb concentration in some cells on measured erythrocyte filterability (deformability) quantitatively, using a nickel mesh filtration system. In this study, we found by increasing the proportion of dense cells in populations of normal or sickle cells and equilibrating these cells with air or carbon monoxide (CO), that intracellular polymerization contributes about 4 times as much as intracellular viscosity and twice as much as abnormal membranes to impaired filterability, and that an increase in dense cells, which contained Hb S polymer due to the high cell hemoglobin concentration (CHC), impaired filtration significantly under air-equilibrated conditions. Moreover, we estimated that the filterability of erythrocytes containing Hb S was extremely sensitive to small amounts of intracellular polymer and that impaired filtration was almost linearly related to intracellular polymer fraction. Thus, the polymerization of Hb S would be expected to be a much more important factor in loss of filtration in microcirculation where oxygen tension is low. These results emphasize the importance of Hb S polymerization in the pathogenesis of sickle cell disease, and will help define the pathophysiology of sickle cell disease in quantitative terms.